sanity: Require gcc 5.0 minimum version

After polling various develoeprs, its clear that many layers are struggling
with gcc 4.8 and its better for the project to adopt 5.0 as a minimum
version at this point in time. We should have technology like
buildtools-extended-tarball available to ensure things still work on Centos 7
and Debian 8.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>